Technical Abstract:
Haemophilus, Actinobacillus and Pasteurella (HAP) are major causes of respiratory and systemic diseases in humans, other mammals, and birds. They exist as mucosal parasites, but are capable of being primary or opportunistic pathogens depending on the carrier status and health of the host. Primary HAP pathogens (i.e., Pasteurella multocida) colonize mucosal lsurfaces and probably invade through respiratory tract-associated lymphoid tissues. After invasion, systemic HAP pathogens utilize lymphatics, the blood system, and other host mechanisms for traffiking. Traverse through the circulating fluids is mainly extracellular. Common sites for HAP commensal residence are the nasopharynx and tonsils. Opportunistic invasion of the respiratory tract from these areas often occurs as a sequelae to ciliated epithelium damage caused by viral, mycoplasmal, or bacterial agents. Complicated diseases often result from interaction of these agents with HAP. A variety of virulence factors may act alone and together to promote HAP adherence to mucosal surfaces and undermine host defenses against colonization. For example, H. influenzae produces pili and filamentous proteins for adhesion and IGA1 protease to counteract secretory immune defense. Capsules on HAP are associated with virulence and promote spread of the organisms. Those occuring on certain P. multocida and H. paragallinarum contain hyaluronic acid or other muco- polysaccharides which mimic host substances and thus are nonantigenic. These and other polysaccharide-containing capsules found among HAP confer resistance to serum killing and/or phagocytosis. Cell and tissue damaging substances are produced by HAP. These include endotoxin, RTX toxins, and a toxin associated with production of turbinate atrophy in swine.